1. Field of the Invention
This invention relates in general to refrigerant expansion devices for use in heat pump systems. More specifically, this invention relates to expansion devices of the type including a moveable piston having a metering port therein which is moveable, responsive to the flow of refrigerant from a refrigerant metering position to a refrigerant by-pass position.
2. Description of the Prior Art
Conventional heat pumps include a refrigeration circuit with a compressor and indoor and outdoor heat exchanger coils which function alternately as a condenser and an evaporator in response to a thermostat controlled valve which reverses the direction of refrigerant flow through the circuit between heating and cooling cycles. During cooling cycles the indoor coil functions as an evaporator, absorbing heat from indoor air, and the outdoor coil functions as a condenser, rejecting heat into the outdoor air. During heating cycles the outdoor coil functions as an evaporator absorbing heat from the outdoor air, and the indoor coil functions as a condenser rejecting that heat to the indoor air for comfort heating.
Since the operating conditions of a heat pump depend upon whether it is in a heating cycle or a cooling cycle, it is known to utilize an expansion device dedicated to each of the operating cycles. The conventional method of accomplishing this was to incorporate a double expansion valve and by-pass system in the supply line connecting the two heat exchangers to accomplish throttling in either direction. In the double expansion valve arrangement, two opposed expansion valves are positioned within the refrigerant supply line between the two heat exchangers. A valve operated by-pass is also positioned in parallel with each expansion valve. When the cycle is reversed, the bypass valves are regulated by a control system to alternately utilize one expansion device and by-pass the other. The double by-pass system thus required relatively expensive hardware to implement and a control system to operate the by-pass valves.
U.S. Pat. No. 3,992,898 issued to the assignee hereof, discloses an expansion device which is capable of metering refrigerant flowing therethrough in one direction and freely by-passing refrigerant flowing therethrough in the opposite direction thereby eliminating the need for the expensive by-pass system. In the device of this patent, the refrigerant metering port is formed in a free floating piston which is mounted within a chamber. When refrigerant flows through this device in one direction, the free floating piston moves to one position wherein the refrigerant flow is through the metering port thereby serving as an expansion device. When refrigerant flows through this device in the opposite direction, the free floating piston moves to a second position wherein the refrigerant is allowed to flow through a number of flow channels formed in the outer periphery of the piston to thereby allow substantially unrestricted flow through the device. This arrangement allows such a device to be used, in combination with a second expansion device of the same design, in a heat pump system to allow the desired expansion of the refrigerant through the system flowing in both cooling and heating directions.
As pointed out above, during heating cycles the outdoor coil functions as an evaporator absorbing heat from the outdoor air, and the indoor coil functions as a condenser rejecting that heat to the indoor air for comfort heating. During the time that outdoor temperatures are around 45.degree. and colder, moisture from the outdoor air is collected on the outdoor coil fins in the form of frost. The frost accumulates progressively in thickness on the fins surfaces thereby reducing heat transfer by blocking air flow therethrough, and by the insulating effect on the fin surfaces.
The frost accumulation is periodically removed by temporarily operating the heat pump in a cooling cycle wherein hot gas discharged from the compressor is circulated to the outdoor coil to heat it for frost removal. A defrost cycle is functionally a temporary cooling cycle. It is common practice to initiate defrost cycles by automatic means responsive to the thickness of frost accumulation, or by an interval timer. Termination of defrost cycles are typically caused by a thermostat which senses temperature rise of the outdoor coil, or its condensate, indicating completion of frost removal.
In a typical prior art heat pump system, each heat pump coil may be provided with its own expansion device of the type disclosed in previously discussed U.S. Pat. No. 3,992,898, to meter refrigerant to the coil which is serving as an evaporator. The device serving the outdoor coil, in heating cycles, provides for metering liquid refrigerant to efficiently meet the circumstances of operation during a range of cold outdoor winter temperatures. For example, at a winter ambient of 25.degree. F. the evaporating pressure in the outdoor coil would be approximately 35 psig, and the condensing pressure in the indoor coil 195 psig, establishing a pressure difference across the expansion device of 160 psi.
The expansion device serving the indoor coil during the summer cooling cycles is selected to meter liquid refrigerant to the indoor coil during a range of summer cooling temperatures. As an example, at 85.degree. F. ambient, the condenser pressure in the outdoor coil would be approximately 250 psig, while the evaporating pressure in the indoor coil would be in the range of 72 psig, establishing a pressure difference across the expansion device of 178 psi.
When a defrost cycle is initiated, refrigerant flow is reversed and circulation of refrigerant in the cooling direction is caused to occur for a set time period, or until a set temperature at the outdoor coil, for example, 80.degree.-85.degree. F., is reached. During defrost operation energy penalties are paid which reduce the operating efficiency of the heat pump system. Specifically, during defrost, electrical energy is being consumed by the refrigeration system to defrost the coil with no resultant mechanical heat from the heat pump system being transferred to the heated area. During defrost, heat is actually being removed from the heated area and transferred to the outdoor coil to melt the frost. Further, during the time of defrost, generally, an electrical resistance back up heating system installed in the duct work is actuated to maintain the heated space at a desired comfort level. As a result, it is evident that, it is extremely desirable to minimize the defrost time of a heat pump system in order to increase the operating efficiency of the system. One common measure of the efficiency of a heat pump system is Heating Seasonal Performance Factor, commonly referred to as HSPF. This term is defined by the United States Department of Energy as "the total heating output of a heat pump during its normal annual usage for heating, divided by the total electrical power input during the same period."
Accordingly, since the electrical input is far more efficient when providing heat through the heat pump system, it is extremely desirable to minimize the length of the defrost cycle.
Typical heat pumps are designed with greater outdoor volume than indoor coil volume. This is done to maximize cooling performance which is typically the major selling feature or purpose of the heat pump. As a result, the circulated refrigerant charge quantity is greater during the cooling cycle than the heating cycle.
Upon initiation of defrost, a heat pump is shifted from a heating cycle to a cooling cycle. One factor effecting the length of the defrost cycle is the time required to get into circulation, the proper amount of refrigerant charge to maximize heat transfer from the conditioned space to the cold frosted outdoor coil. When a defrost cycle is initiated, by establishing a temporary cooling cycle under typical winter ambient conditions, the condensing pressure in the outdoor coil is the maximum pressure available for delivering refrigerant from the outdoor coil to the indoor coil through the cooling expansion device. Under such circumstances, the cooling expansion device exhibits a high resistance to flow thereacross because it is designed to control refrigerant flow under a pressure differential in the range of 178 psi as shown in the example given above. Under such circumstances, the compressor is usually required to reduce the pressure in the indoor coil to a very low pressure to establish a pressure differential capable of feeding the indoor coil. In some systems, under certain circumstances, a satisfactory defrost cycle cannot be accomplished with the cooling expansion device serving as the defrost expansion valve.
It has been recognized that during defrost operation, the difference between the high and low pressure sides in a heat pump system is so small that optimal refrigerant circulation is not guaranteed. One approach to solving this problem has been to provide a solenoid actuated by-pass arrangement which provides a large, very low resistance, path by-passing the cooling expansion valve during defrost operations. Such a by-pass allows refrigerant, previously stored in the accumulator during the heating cycle, to be quickly withdrawn and put into circulation where it may deliver heat to the outdoor coil thereby reducing defrost times.
Such a solution to the defrost performance problem however is expensive and represents a step backward in that one of the significant advantages of the combination expansion device was the elimination of the plumbing, valves and controls associated with the by-pass systems.